1. Field
The present disclosure relates to systems and methods for creating and displaying autostereoscopic three-dimensional images using a holographic optical element.
2. Background Art
Stereoscopic display devices separate left and right images corresponding to slightly different views or perspectives of a three-dimensional scene or object and direct the images to a viewer's left and right eye, respectively. The viewer's visual system then combines the left-eye and right-eye views to perceive a three-dimensional or stereo image. A variety of different strategies have been used to capture or create the left and right views, and to deliver or display them to one or more viewers. Stereoscopic displays often rely on special glasses or headgear worn by the viewer(s) to deliver the left and right images to the viewer's left and right eyes. These have various disadvantages. As such, a number of strategies have been developed to provide autostereoscopic displays, which deliver the left and right images to corresponding eyes of one or more viewers without the use of special glasses or headgear.
One strategy for displaying an autostereoscopic image uses a parallax barrier. This method uses interlaced left and right images and places a layer of material with very fine slits at a precise distance from the image plane of a flat panel display (typically LCD), relying on parallax to separate right-eye and left-eye viewing perspectives so that each eye sees the corresponding left/right image. However, horizontal resolution and light output are adversely impacted with this approach, and the “sweet spot”, or zone where one can best visualize a stereoscopic image, is very small.
Another attempt at providing an autostereoscopic image uses a Fresnel lens to direct reflected light from left and right stereo video sources to corresponding left-eye and right-eye viewing locations. While the use of a Fresnel lens enables lower volume and weight for large aperture, short focal length lenses, image quality or resolution is reduced. As such, three-dimensional imaging systems based on parallax barriers and Fresnel lenses, as well as those using lenticular sheets, have generally fallen short of user expectations.
Various strategies for creating an autostereoscopic display have incorporated a holographic optical element (HOE) that is made by holographically recording an image of a diffuse viewing zone or eyebox created by a rectangular ground glass plate. During replay, the HOE is co-illuminated with left and right images from different horizontal angles and redirects the images to corresponding left-eye and right-eye viewing zones or eyeboxes for viewing by the left and right eyes of the viewer. In one approach, the HOE is recorded with a single monochromatic light source, such as a laser, with the ground glass plate positioned at the achromatic angle to create a rainbow hologram. During replay with broadband (white) light, the image of the ground glass plate is dispersed along the achromatic angle. If the ground glass plate is longer than the dispersion, there will be an area equal to the difference between the object and the dispersion where all colors of the spectrum overlap to provide a full color display. However, the region of color fidelity is generally of very limited extent such that any vertical movement by the viewer results in color shifting and poor color reproduction of the projected image. Such effects are distracting and make this approach unsuitable for a variety of applications, particularly where color fidelity is desired, such as in medical imaging, entertainment, and a variety of other applications.
Attempts to solve various problems associated with an autostereoscopic display system incorporating a HOE have included the use of multiple monochromatic sources implemented by lasers of different emission wavelengths to record the HOE. In various approaches, three or more different wavelengths are used during the recording process that generally include at least some wavelengths of red, green, and blue light to improve the color fidelity within a larger viewing zone of the display during playback. This introduces numerous challenges due to the frequency (or wavelength) sensitivity/dependence of the recording medium and various optical elements used in both the recording and playback of the HOE. A holographic recording medium having low scattering and capable of high resolution with appropriate sensitivity to the recording wavelengths is generally needed for the master or original recording. One solution is to use different media for the different wavelengths with the media layered or sandwiched together to produce the HOE viewing screen. However, this approach introduces additional complexities associated with having multiple recording set-ups, precise control of environmental conditions during multiple exposures, alignment or registration of the layers, and the like. More recently, the availability of a single panchromatic medium with suitable sensitivity and scattering characteristics for the recording wavelengths, such as a high resolution silver halide emulsion, for example, has facilitated recording in a single layer. The emulsion may be exposed using multiple wavelengths either simultaneously or sequentially during the recording process and developed using known holographic developing techniques. Use of a single recording medium and simultaneous recording of multiple wavelengths greatly simplifies the recording and developing process.
During recording of an HOE, a diffuser, which may be implemented by a ground glass plate, having the desired geometry of a viewing zone or eyebox is illuminated by an object beam passing through the diffuser and interfering with a reference beam to create an interference pattern recorded in the panchromatic medium. The laser beams used during recording generally have a non-uniform intensity distribution or profile with higher intensity at the center of the beam that tapers off toward the edge of the beam. The intensity profile or distribution may vary depending on the operating mode and type of laser. For example, a helium-neon (He—Ne) laser used to provide one wavelength of red light generally produces a circular beam with a fundamental transverse mode (TEM0,0) with an approximately radially symmetric Gaussian profile, while a neodymium:ytterbium-aluminum-garnet (Nd-YAG) laser used to provide a second wavelength of green light generally exhibits more of an elliptical beam with a Gaussian intensity profile that varies asymmetrically in the vertical and horizontal directions. The present disclosure recognizes that such non-uniform illumination of the HOE during recording may result in corresponding intensity variations or vignetting during playback of the HOE, i.e. the autostereoscopic image generated by the HOE appears brighter in the center of the screen and progressively darker around the edges.
One strategy for improving uniformity of illumination is to overfill the plate or screen during recording, which effectively captures the more uniform intensity near the center of the object and reference beams. This generally requires higher power lasers and associated optical components capable of accommodating the higher intensity beams. Alternatively, or in combination, longer exposure times may also be required, which are more susceptible to noise from vibrations or other environmental factors during exposure.
Another strategy for providing more uniform illumination is to use pulsed lasers to record a composite HOE having individual pixels that are each a separate hologram. This may provide various advantages relative to an HOE recorded in a single simultaneous exposure (or sequential exposures) with continuous wave (cw) lasers, such as allowing adjustments to be made on a pixel-by-pixel basis to provide a more uniform HOE from edge-to-edge. However, the pixel size and fill ratio or packing density limits the resulting resolution, which may not be acceptable for smaller screens for use in personal entertainment or gaming, or in more demanding applications where high resolution is desired, such as in medical imaging, for example.
In addition to high resolution, various applications may demand good color fidelity and preservation of aspect ratio so that viewed objects and distances are accurately depicted by the autostereoscopic display. For example, in medical imaging applications, a surgeon may rely on the color of tissue to distinguish between healthy and diseased tissue. Similarly, accurate manipulation of surgical tools demands little or no distortion in the images projected onto and by the HOE screen in the autostereoscopic display. Such requirements present additional challenges for autostereoscopic display systems, which may use various types of projectors to illuminate the HOE screen with the left-eye and right-eye images. Color fidelity may be affected by the wavelength(s) of the light source used by the projectors relative to the light sources used in recording the HOE, as well as the stability of the emulsion during developing and after mounting the HOE. Various types of image distortion or optical aberration induced by the projection optics, such as pincushion, barrel, or mustache, as well as keystone or tombstone effects created by positioning of the projectors at an angle relative to the HOE screen generally also need to be corrected. Depending on the particular projectors being used, some digital image correction may be provided, although this generally results in reduced resolution of the autostereoscopic system.
To maintain the three-dimensional image when viewing an autostereoscopic display, the left eye and right eye of the viewer must be aligned within corresponding viewing zones, eye boxes, or sweet spots of the display, and within a predetermined range or distance from the display, which depends on the particular type of screen and recording process used for the screen. To provide acceptable image contrast (corresponding to efficient light reflection/transmission to the eye boxes) generally requires eye boxes of fairly limited horizontal extent, which effectively limits viewer movement to maintain a three-dimensional image. Various strategies for eye tracking or head tracking have been developed to improve viewer mobility while maintaining alignment of the viewer's eyes with the eye boxes to maintain stereopsis and perception of a three-dimensional image.